My mom has a sexually transmitted disease? Did my dad cheat on her? Help!?

My mom discovered today that she has lesions on her cervix due to Human papillomavirus which is a sexually trasmitted disease. My parents have been married for 30 years and they are a lovely and happy couple. But could this news mean that my dad has been cheating on her and passed her the disease? Or is there another way to be infected? This is really bothering me a lot and I'd love if you could answer me. Thank you.